Radio frequency transformer



Sept. 13, 1960 H A, G, STONE 2,952,822

`RADIO FREQUENCY TRANSFORMER Filed May l, 1959 FREQ. IN MEGACYCLES ATTORNEY.

lUnited States Patent C ice RADIO FREQUENCY TRANSFORMER.

Harrison A. G. Stone, Belmar, NJ., assgnorto the United States of America as represented by the Secretary of the Army Filed May 1, 1959, Ser. No. 810,514

9 Claims. (Cl. S33-32) (Granted under Title 35, U.S. Code (1952), sec. 266) The invention described herein may be manufactured and used by or for the Government for governmental purposes, without the payment of any royalty thereof.

This invention relates to a coupling device having inherent capabilities for eciently transferring energy from one system to another wherein the systems are themselves each unbalanced, that is to say, in each system one terminal is at ground potential while the other terminal is at some higher potential.

The invention accomplishes the transfer of energy while providing a substantial increase in impedance. The impedance gain may be as high as 1 to 4.

' A vital feature of the transformer is its ability to function over an extremely wide band of frequencies. For example, from 40 mc./s. to 400 mc./s. have been covered with no greater deviation than an -allowable 2 to l standing wave ratio. Moreover, this transformation may be made to occur over a wide band of frequencies with low losses, and without the use of tuning or adjustments of any kind over the entire range of frequencies.

The invention provides a successful solution for a dicult problem which has heretofore been circumvented but not solved by the use of ineiicient devices. 'Ihe problem in its broad aspect is to provide ecient coupling between two circuits, the terminals of each of which are at different potentials. The operating characteristics of the coupling device of the invention are such that, not only is eflicient coupling achieved, but the same favorable transfer of energy takes place with substantially uniform results spread over an extremely wide band of frequencies. A still further land no less important feature of the invention is the facility with which the device may be designed to provide high or low impedance4 ratios with which to match impedances of the one circuit to that of the other.

All of the above characteristics are combined with the exceptional ability of the device to function with substantially uniform low loss throughout a wide band of fre, quencies.

The basic structure of the transformer includes. a pair of spaced coil forms of insulating material mountedwithin a shield. The coils are wound upon the supports in helical conlguration and consist of a pair of slightly separated parallel conductors which constitute biiilar windings. The windings are arranged to present unbalanced input and output terminal pairs for connection between two unbalanced electrical systems to be coupled by the transformer. A more detailed description. will appear hereinafter., Itis a primary object of the inventionl to provide a radio frequency transformer for eciently transferring energy from one circuit to yanother wherein both circuits present electrically unbalanced pairs of terminalsV to the transformer terminals.

'A further object of the invention is to provide a radio frequency transformer which functions throughout a wide lband of frequencies without deviation from a highly etli- -cient standard of performance.

25,952,822 Patented Sept. 13, 1960 A further object of the invention ispto provide-a wide-l bandV unbalanoe to unbalance radiofrequency transformen capable of providing an impedance ratio between the coupled circuits as high as l to 4.

A further object of the invention is to provide a radio transformer in which its inherent capabilities are effective without tuning or adjustment.

Other objects and features ofl the invention will more fully `appear from the following description and-will be particularly pointed out inthe claims. v

To present Ia better understanding of theinvention a specific embodiment thereof will be described andy illustrated inthe accompanying `drawings wherein:

Fig. 1 is a side view of the device partially in cross sec-I tion. l

Fig; 2 is a `view similar to Fig. 1' with the device rotated 9Q. l

Fig. 3 is a top viewV of the device of Fig, l along line 3 3. Y u

Fig. 4 is. a circuit diagram of thedevice ofthe invention.

Fig. 5 is a detailed view of the transformer construetion. 1 Y Figo is a graph showing typical electrical performance of the device.

The structural features of theinvention provide dexia bility in designing the transformer to function in a wide range of frequency bands and toaceurately. meet-and in` denitely. maintain its operating chalacteristics..` The transformer windings are inclosed in a metallic, shield 5 which may be of, any suitabley construction. A desirable constructionis shownV in Fig.` 1 wherein the, shieldisa hollow metallic cylinder which. fits closely upon` a metallic circular base member `6 and extends upward to a point above the 4coils and desirably is secured to the base'by screws. 7 or other suitable. means. r[he base serves asV the lower inclosure for the.y shield aswell as the main support for the transformer, windings as will appear hereinafter.- The top Of. the shieldis closedy by means of a disc 8 ofA in..- sulating material securedV in the upper end of. the. cylinder by screws 9 or other suitable. means. g

The windings ofthe transformer. are supported upon a pair of coil forms 10 and 11 of low loss insulating material such as Teon polystyrene'or Lucite. The coil forms are identical and, are rigidly secured to the base. 6in spaced relation with their `axes parallel `and equally distant from the shield. The forms are generally spool-like in conf figuration having a cylindrical body 12 to receive. the coil windings and top and bottom end portions of enlarged diameter. The bottom end portions 13 are desirably'held. rigidly within recesses formed inthe base 6 and secured by screws 14. The top'end portions v15 are recessed to tightly receive output terminals 16 and 17V which extend upward through the disc Sand -are provided with suitable means for connection to a circuit. l

The input to the transformer is supplied by a coaxial cable which has detachable connection with a Conventional coaxial plug connector 18 which is received an aperture drilled through the center of the. base 6j.. The plug conneotoris fixed .to the base. by screws or any suit,- able means which provides good electrical connection between the base and the outer member `19v of the coaxial system.

The inner member V20' of the coaxial plug 1S projects into the space within the shield and provides the. high voltage connection for the while. groundfpoten'.- tial is maintained throughoutthe base, the. Shielil andthe outer member of thecoaxial input cable.' vOther means than a coaxial cable may, hoxveve'r,A be provided tolconduct input energy tothe transformer. f It is 'alsopossible to `provide coaxial transfer of energy trom the Voutput terminals.

Y An additional element is provided in the form of shnntin'gfbus bar Z1 which. is rigidly electlically Connected at one end to the basemember 6 and may desirably be fastened by one of the screws which secure the plug 1,8 tothe base. The bar 2.1 extends upwardpar-allel to the axes of the coil terms and is spaced 'Yequaldistances therefrom. The upper end of the bar terminates at or slightly beyond the upper end of the coil forms and'is mechanically supported at this point by .insertion of the edges of the bar intoslots, provided for the pui-pose in the upper enlarged portionsr 15 of the coil forms. 'I'he provision of the :above supporting means Afor the bar 21 is not essential but is desirable. f

The .transformer windings are of special design. Each coil formhas two vseparated conductors wound thereon ratio of 2.0 throughout a frequency range Vof from 40 rnc/s. to 400 mc./s. In the test, the transformer was connected between two unbalanced electrical systems.

Relative to the theoretical considerations concerning the operation `and designpfthedevice it should be observed that .the structure departs from the generally accepted design for such apparatus in that one ofthe windings of one biiilar coil is shunted by a low impedance in spaced helical configuration to form bifilar windings Y whichare connected to provide unbalanced input and unbalanced output. 'Ivheuniformity of the spacing of .the conductors: is held to close tolerance. Also the pitch of the helix is accurately uniform and held to a predetermined value commensurate with the number of turns member 21 which would appear to render winding ineiective. It has been found, however, thatthe device will not function without .this shunted winding. l

An explanation of the operationof the device can not be based solely upon lumped constant circuit parameters. 'I'.he Vdevice must instead be treated `as a distributed line transformer. 'I'his .assumption then implicitly implies that .the behavior of the device is not that of a distortionless transmission line.

The apparent length of the device must be considered constant in .terms of an electrical wavelength throughout the range of its operation to explain its operability over a l0 to 1 frequency range. A precedent for this restliction is exhibited by `a To mode helix antenna and also by a helical center conductor coaxial .transmission Vline.- The equations which describe the operation of the To mode'helix provide a lead inthe approach lto a` setk of design relationships which can be employed-in predicting thestructure and operation of ythis device of the invention.

'Ihe indicated relationships which should be considered 'Y are as follows. The spacing of the conductors forming One groove is cut .to accurate depth throughout the length ,p

ofthe coil form with the lathe adiusted to provide the required helical pitch. The lathe cutting tool is then adjusted along Ythe lathe axis-the Ydistance of the spacing between 'the parallel conductors and the secondY groove is cut.

forms `11Y and 12 to receive the transformerV desirably are formed with sloping side walls and a flat bottom wall similar tothe shape of an Acme type screw thread and so proportioned that the wire received therein will closely approach but not contact the bottom wall thereof while contacting only the side walls.

' ,The conductors, which desirably may be copper wire of the required size, are then wound upon the forms in the grooves thus cut and held .therein in any .suitable manner, such as cementing .them in place with insulating cement. 'I'he ends of the wires are then connected to the .terminals of the device by soldering or by other low impedance connecting means.

-The inner conductor 20 of the coaxial cable is connected to the lower end of one of the windings, for example, the winding 22 on coil form 11. The upper end of .this winding-is connected tothe upper end of the Winding 25 on. coil form 210 while the lower end of the be 3 inches. This device was intended to winding 25 is connected to .thezouter member 19 thru the .bar 21 and base 6 of the coaxial input connector which is at .ground potential. The inner member 20 of the connector is :also connected to .the lower Yend of the 24 while its upper end is connected Ito the output terminal A16. The winding 23 is connected at its lower end t'o the vgrounded outer member 19 of the coaxial cable. The upper end of this winding is connected to the output 'terminal 17. The ends of the shunting bar 2'1 arefconnected to opposite ends of the winding 23;

Il'ransformers constructed in accordance with 4the in# vention, have been made to cover various bands of tfre.- quencies having a l0 to l range. Test data obtained from la device constructed in accordancewith the in- `vention exhibited a 'gain in impedance of 1 to 4 and perdevice `Aflmctioned well an allowable standing wave a pair of biiilar windings must be chosen on the basis of a pair cf windings semi-immersed in a dielectric mediumwhose characteristic impedance should be arranged-to meet the krequirement that it is equalV to the square root of the product `ofthe terminal input and terminal output impedances which the device sees. The spacing between the twoforms is chosenonthe basis of an impedancev relationship whichfis that of a balanced Y Y Y c l pair of current'sheets in a grounded shield. On It has YVbeen found that the slots :formed in the coil basis predictions were made for a specific device which were borne out quite accurately in practice.

lBased on the above considerations the predicted parameters for the speci-lic device indicated 6.12 turns per inch of number 18 wire upon a -coil form which was capable of receiving .a coil 3.81 inches long. Thus the total number of turns predicted was 23.3. 'The spacing of the wires in-their .bilar relation was .034 inch, the spacing of the coil forms was determined to be .875 inch ibetween centers and the diameter of the shield can was to operate at from 2O to 200 megacycles.

When built and adjusted for best operation the device employed 23 turns of number 18 wire on eachvcoil form atvthe ybiilar spacing indicated above. The coil form had 6 turns per inchr'and was 4 inches long. The shield can was found to provide optimum results when 2.8 inches in diameter instead of` 3 inches. This device operated at a voltage standing waveratio of less than 2'to l from 20Yto 200 megacycles. The operating characteristics followedfgenerally that predicted and showed the same highly'desirable ,capabilities as that of the device whose output is illustrated in fFig. 6. l Other unbalance type transformers were successfully built in accordance withthe invention to function at various frequency ranges and to function in connection withvarious input and load impedances.

What is claimed is: Y

- 1. A Aradio frequency transformer comprising iirst and second coil forms each having biiilar windings thereon whichl constitute the windings of the transformer, a metallic shield in closing ,saidv windings, a pair of input and a pair of output terminals, one coilV on said first coil form and' one coilfon said second `coil form being connected in series between said input terminals, the vother coil on s'a'id second coil form being connected in series between the other input terminal `andvoneoutput-terminal, the other coil on said first coil form being connected between ground potential and the second output terminal and a shunting bus bar having its ends connected to the ends of the said latter coil.

2. A radio frequency transformer for coupling two unbalanced electrical systems comprising first and second coil forms of low loss insulation each having two separated parallel conductors wound thereon in spaced helical configuration which constitute the windings of the transformer, a metallic shield inclosing said windings and having a ground potential connection, a coaxial connecting means for supplying input energy and a pair of output terminals, one coil on said first coil form and one coil on said second coil form being connected in series between the inner and outer conductors of said coaxial connector, the other coil on said second coil form being connected in series between the inner conductor of said coaxial connector and one of said output terminals, the other coil on said second coil form being connected in series between the outer conductor of said coaxial connector and the other output terminal, the other coil on said first coil form being connected between ground potential on the second output terminal and a shunting bus bar having its ends connected to the ends of the said latter coil.

3. A radio frequency transformer for coupling two unbalanced electrical systems comprising first and second coil forms of low loss insulation each having two separated parallel conductors wound thereon in spaced helical configuration which constitute the windings of the transformer, a metallic base member upon which said coil forms are rigidly secured in spaced parallel relation, a metallic shield inclosing said coils and having electrical connection with said base, said base and shield having a ground potential connection, a pair of input and a pair of output terminals, one coil on said first coil form and one coil on said second coil form being connected in series between said input terminals, the other coil on said second coil form being connected in series between the high potential input terminal, `and the high potential output terminal, the other coil on said first coil form being connected in series between the ground potential input terminal and the ground potential output terminal and a shunting bus bar having its ends connected to the ends of said latter coil.

4. A radio frequency transformer comprising first and second coil forms supported in spaced parallel position, each coil form having formed therein a pair of separated parallel surface grooves of spaced helical conguration extending substantially the full length of the forms, pairs of wires for each form received in said grooves to form the windings of the transformer, a metallic shield inclosing said windings and having a ground potential connection, a pair of input and a pair of output terminals, one coil on said rst coil form and one coil on said second coil form being connected in series between said input terminals, the other coil on said second coil form being connected in series between the high potential input terminal and the high potential output terminal, the other coil on said first coil form being connected in series between the ground potential input terminal land the ground potential output terminal and a shunting bus bar having its ends connected to the ends of said latter coil.

5. A radio frequency transformer according to claim 4 and wherein the input terminals thereof are in the form of a coaxial connecting means.

6. A r-adio frequency transformer according to claim l and in which the impedance r-atio thereof is at least 1 to 4.

7. A radio frequency transformer according to claim 3 and wherein the said shunting bus bar is parallel to the said coil forms and is rigidly supported at one end on said base member and at its other end is attached to the upper end portions of the said coil forms.

8. A radio frequency transformer comprising first and second coil forms of low loss insulation each having two separated parallel conductors wound thereon in spaced helical configuration to constitute the windings for the transformer, means to rigidly secure the conductors in accurate uniformly spaced relation throughout their length, a metallic shield inclosing said windings and having a ground potential connection, a metallic base member, means to secure said coil forms in parallel spaced relation on said base, a metallic shield inclosing said windings and electrically connected to said base, Va coaxial connecting means for supplying input energy, the inner conductor of said means extending centrally through said base to a point close to said windings, a pair of output terminals supported on said coil forms, one coil on said first coil and one coil on said second coil form being connected in series between the said inner connector of said coaxial connecting means and the outer conductor thereof, the other coil on said second coil form being connected in series between the inner conductor of said coaxial connector and one of said output terminals, the other coil on said first coil form being connected in series between the outer conductor of said coaxial connector and the other output terminal, and a shunting bus bar connected at its ends to the ends of said latter coil.

9. A radio frequency transformer according to claim 8 together with a disc of insulation closing the top of said shield and through which said output terminals extend.

References Cited in the file of this patent UNITED STATES PATENTS 2,791,585 Delong Nov. 20, 1956 FOREIGN PATENTS 590,203 Great Britain July 10, 1947 

